Technical Field
The present invention relates to memory systems and in particular to systems and methods for writing data to magnetic memory systems using spin-polarized electron beams.
Background Art
The predominant mass storage device in conventional computer system is the hard disk drive. Hard disk drives are relatively large, electromechanical devices that can store tens of gigabits of data. The stored data is accessed through a read/write head that rides on a cushion of air above the rapidly rotating disk. The read/write head is moved radially to access data in different tracks of the rotating disk. Data transfer is limited by the speed at which the disc rotates and the speed with which the read/write head is positioned over the required track. Even with the fastest devices, these times are on the order of hundreds of microseconds, because relatively large mechanical motions are involved. This time scale is at least five orders of magnitude slower than the nanosecond time scales at which processors operate. The discrepancy can leave the processor starved for data.
One proposed data storage system that has both higher data densities and faster access times than currently available mass storage devices employs spin-polarized electrons to read data from and write data to a storage medium. Electron beams can be manipulated by charged-particle optics, which operate on time scales closer to those seen in processors. U.S. Pat. Nos. 5,546,337 and 5,604,706 describe systems that employ spin-polarized electrons to transfer data to and from a storage medium. The disclosed systems scatter spin-polarized electron beams from the magnetic moments associated with different storage locations on the medium to read data from and, arguably, to write data to these locations.
One problem with the disclosed system is that the characteristics of the storage media that provide desirable magnetic properties also make it difficult to alter these magnetic properties, i.e. to write data, using electron beams. A typical storage medium includes a relatively thin layer of a magnetic material, such as iron (Fe), deposited on a layer of a conductive, non-magnetic material. This combination of material layers creates a quantum well. Using a thin layer of magnetic material forces the easy axis of magnetization out of the plane of the magnetic layer. This perpendicular magnetization supports denser packing of the magnetic domains that represent individual data bits, e.g. the storage locations.
The thin magnetic layer of a storage location is illuminated with a spin-polarized electron beam emitted from a source located a few millimeters above the storage medium. The source-to-medium separation provides time to steer the beam to the targeted storage location. However, this geometry also delivers the beam electrons to the magnetic layer at normal or near normal angles of incidence, i.e. parallel to the thinnest dimension of the magnetic layer. The volume of the magnetic layer sampled by the electron beam ("interaction volume") is thus relatively small, and the number of electrons in the magnetic layer ("target electrons") to which the beam electrons couple is similarly small.
An additional problem is that electron beam sources produce electron energies of several electron volts (eV). At these energies, the probability of an incident electron being bound by the quantum well formed by the magnetic layer is greatly reduced. The combination of the thin layer of magnetic material, the normally incident beam, and the high electron energy thus limits the strength of the coupling between the spin-polarized electron beam and the target electrons.
In order to flip the spin state associated with a storage location, the relative number of spin up and spin down electrons must be reversed before spin relaxation mechanisms restore the status quo. In ferromagnetic materials, exchange interactions among the electrons make the dominant spin state more stable than the minority spin state. If a relatively small portion of the dominant spin electrons interact with the spin-polarized electron beam, any spin flipping triggered by the beam will be ameliorated by spin relaxation mechanisms.
Effective spin flipping thus requires the incident beam of spin-polarized electrons to interact with a large number of electrons in the magnetic material (target electrons) over a relatively short time. The electron beam must effect a critical number of target electrons within a to spin relaxation time, or else the dominant spin state will reestablish itself.
Systems have been developed that allow the coupling between spin-polarized electrons and the target electrons in the magnetic material to reverse the majority spin state. For example, scanning tunneling microscopes (STMs) employ scanning tips that are positioned within a few Angstroms of the surface of the magnetic layer. The scanning tip may be modified to cause spin-polarized electrons to tunnel from the tip into the medium. STMs emit electrons into the target material with energies substantially below 1 eV. As a result, the spin-polarized electrons tend to become trapped by the quantum well formed by the magnetic layer and move laterally along the layer, allowing each incident electron to couple to many target electrons before it exits the magnetic material. STM's can provide very high spin-polarized current densities, e.g. several micro Amps per square nanometer. The combination of quantum well trapping and very high current density has been demonstrated to reverse the magnetic sense of a thin magnetic film.
The STM geometry is not suitable for storage media. For example, the close proximity of the scanning tip to the surface limits the area of the storage medium that may be scanned by deflecting the electron beam. STM scanning techniques, which translate the entire STM apparatus relative to the medium, are too slow for processor applications.
The present invention address these and other problems associated with writing data to magnetic media using spin-polarized electron beams.